tcp: Fix data-races around sysctl_tcp_fastopen_blackhole_timeout.

While reading sysctl_tcp_fastopen_blackhole_timeout, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its readers.

Fixes: cf1ef3f0 ("net/tcp_fastopen: Disable active side TFO in certain scenarios")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_fastopen.c

@@ -489,7 +489,7 @@ void tcp_fastopen_active_disable(struct sock *sk)
 {
 	struct net *net = sock_net(sk);
 
-	if (!sock_net(sk)->ipv4.sysctl_tcp_fastopen_blackhole_timeout)
+	if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen_blackhole_timeout))
 		return;
 
 	/* Paired with READ_ONCE() in tcp_fastopen_active_should_disable() */
@@ -510,7 +510,8 @@ void tcp_fastopen_active_disable(struct sock *sk)
  */
 bool tcp_fastopen_active_should_disable(struct sock *sk)
 {
-	unsigned int tfo_bh_timeout = sock_net(sk)->ipv4.sysctl_tcp_fastopen_blackhole_timeout;
+	unsigned int tfo_bh_timeout =
+		READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_fastopen_blackhole_timeout);
 	unsigned long timeout;
 	int tfo_da_times;
 	int multiplier;